Synergistic decomposable set-inhibitors for plaster of paris



United States Patent 3 294,087 SYNERGISTHC DECOR HQSABLE SET-INHIBITORSFOR PLASTElR 0F PARIS David F. Smith, 120 Grove St., Bay Head, NJ. 08742No Drawing. Filed May 2, 1966, Ser. No. 546,572 6 Claims. (Cl. 1128-91}This invention relates to materials which show a synergistic action indelaying the set of plaster of Paris in contact with water and which areremovable by heating. Such materials are particularly useful in makingthe so-called plaster of Paris bandages used in orthopedics for formingcasts over parts of the human or animal body where immobilization orsupport is required.

The modern type of such bandage, called hard-coated, is usually made byspreading upon a gauze, crinoline, or other flexible, inert carrier, aslurry of powdered plaster of Paris in an inert non-aqueous liquid, orwater containing a set-inhibitor which is removed or destroyed when thecoated carrier is subsequently heated to remove the slurry liquid. Foreconomic and other reasons, a water slurry is desirable in which casethe set-inhibitor described in US. Patent No. 2,557,083 by Eberl may beused. This ammonium borate set-inhibitor is largely decomposed when theslurry is heated, leaving boric acid or an ammonium borate of lowammonia content which does not largely delay the set of the final dryplaster. However, in normal operation I have discovered that at leasttraces of the set-inhibitor do apparently remain in the dry product.Since minor amounts (very small concentrations in the slurry liquid) ofborate can delay the set for many hours or even days, even traces ofborate can delay the set of the dry plaster for at least several minutesand such a delay is important to avoid since the plaster bandage isrequired to set in 2 to 8 minutes when subsequently wet in waterpreparatory to use in making a cast. For example, a plaster bandage madeaccording to the Eberl method, using about 1% K 80 set-accelerator basedon the weight of plaster, sets in 3 /2 minutes while a similar bandagemade using 82% aqueous methanol sets in 2 /3 to 2 /2 minutes; thesetting times being determined under strictly comparable conditions. Inthe latter case there is no possibility of any set-inhibitor remainingin the dry bandage. Such a difference in setting-time is important sinceit can represent the diiference between extra-fast setting (1-3minutes), and fast setting bandage (3-8 minutes) as defined in federalspecification GG-B-10ld; June 2, 1959. As a matter of fact, widely usedbandages of the fast and extra-fast type ditfer in setting-time undersome practical conditions of use, by less than 1 minute.

The delay of the set of the dry bandage, of course, depends upon theratio of the weight of set-inhibitor left in the product to the weightof plaster; and the greater the proportion of set-inhibitor to plasterin the slurry, the greater will normally be this proportion in the dryproduct. One can, of course, reduce the proportion of set-inhibitor toplaster by using less set-inhibitor, but the Eberl method has a definitelower limit in proportion of set-inhibitor to plaster since the life ofthe slurry thereby decreases to a practical lower limit.

My invention resides in my discovery of a practical method of overcomingthis diificulty with the Eberl method. In addition to ammonium borate inthe slurry, I use an excess of ammonia which further delays the set ofthe slurry without causing any detectable delay of the set of the finalproduct since the NH is readily and completely volatilized upon heatingand drying of the slurry. More particularly, my invention resides in thediscovery that there is a marked synergestic action between ammoniumborate and ammonia in delaying the set of plaster in the slurry, asillustrated in the following examples.

Example 1 A solution was made containing the following materials: GramsWater 200 Halo, 1.15 29.4 wt. percent aqua NH 4 K 4.5

The above solution was mixed with 350 grams powdered plaster of Pariswhich did not set until about 4 hours after it was mixed with thesolution. (Within practical limits the amount of plaster relative tosolution is immaterial from the standpoint of life of the slurry since agiven solution will cause setting of a smal amount of plaster in aboutthe same time as with a large amount of plaster, the composition of thesolution being determining. The amount of plaster added is controlled bythe thickness of slurry required for the coating method used.

Example 2 A solution was made using the following materials: Grams Water176.7

H3BO3 1.5 29.4 wt. percent aqua NH 27.3 K SO, 4.5

The above solution was mixed with 350 grams of another part of the sameplaster of Paris used in Example 1. The plaster did not set until about5 hours.

Example 3 A solution was made containing the following materials:

' Grams Water 200 H BO 0.35 29.4 wt. percent aqua NH 4.0 K 80 4.5

After the above solution was mixed with 350 grams of another part of thesame plaster of Paris used in Example 1, the plaster set in 7 minutes.

Example 4 A solution was made containing the following materials: GramsWater 136 H BO 0.35 29.4 wt. percent aqua NH 68 K 50 4.5

After the above solution was mixed with another part of the same plasterof Paris used in Example 1, the plaster set in 24 minutes.

Example 5 A solution was made containing the following materials: GramsWater 79 29.4 wt. percent aqua NH K 50 4.5

After the above solution was mixed with 350 grams of another part of thesame plaster of Paris used in Example 1, the plaster set in 2% hours.

Example 6 A solution was made containing the following materials: GramsWater 79 H BO 0.35 29.4 wt. percent aqua NH 125 K 80 4.5

by Eberl.

After mixing the above solution with 350 grams of another part of thesame plaster of Paris used in Example l, the plaster set in 19 hours.

Example 1 (0.58% NH or 2% aqua ammonia) is illustrative of the Eberlprocess although usually he uses higher concentrations of borate inorder to obtain longer slurry life. Example 2 shows a higher ammoniaconcentration (3.9% NH or 13% aqua ammonia) than used The slurry life ofExample 2 is somewhat longer than that of Example 1 but only slightlylonger. Example 3 illustrates the very short slurry life with reducedborate concentration. Example 4 shows a measurable increase in slurrylife over that of Example 3 at the same borate concentration, but evenwhen the NH, concentration is increased to 9.8% NH (33% aqua ammonia)the slurry life is only 24 minutes. Example 5 shows a moderate slurrylife when the NH concentration is increased to 18% (61% aqua ammonia),without borate. However, in Example 6 where the NH concentration is thesame as in Example 5, but the borate (rboric acid) concentration is thesame in Examples 3 and 4, a very long slurry life is obtained. ThusExample 3 (or even Example 4) shows the very short slurry life due toammonium borate at a low concentration and Example 5 shows the moderateslurry life due to 18% NH;, without borate while Example 6 shows aslurry life much greater than can be explained by the sum of the setdelay of the borate alone (Example 3) and the NH alone (Example 5). Thusthere is marked synergism in the set delay of :borate and NH in the samesolution. Thus the use of NH permits the use of much lower borateconcentration in the slurry (and thus a lower ratio of borate toplaster) while still obtaining sufiiciently long slurry life.

The slurry of Example 1 when spread upon crinoline and dried at 190 to250 F. gives a bandage setting in about 3 /2 minutes, whereas, similaruse of the slurry of Example 6 gives a setting time of 2% minutes undersimilar conditions of test.

I may use boric acid in amount from 0.1 to about 1 percent by weight ofthe plaster of Paris and a concentration of NH in the slurry liquid fromabout 8 percent NH to about 2 3 percent NH based on weight of slurryliquid (H O plus NH Rather than using a high concentration of NH alone,there is advantage in using at least 0.05 percent H BO based on theweight of plaster so that as the slurry liquid is evaporated, the NHlargely lost and the borate thus concentrated, if the partially dryslurry is accidentally subjected to a temperature below about 190 F.(below which plaster of Paris alone will hydrate with water vapor atabout atmospheric pressure) the plaster is protected from setting.

My slurry liquid may contain from about 0.5 to 2.5 percent K 50set-accelerator based on the weight of plaster and from 0.2 to 2 percentbased on the weight of plaster of a bonding agent comprising cookedstarch, dextrin, polyvinyl acetate emulsion, methyl cellulose,hydroxyethyl ethyl cellulose, hydroxypropyl methyl cellulose andmixtures of these.

Having thus described my invention, what I claim is:

1. A settable plaster of Paris mix comprising major proportions ofpowdered plaster of Paris and water and minor proportions of (1) boricacid in amount from about 0.05 to about 1 percent of the weight ofplaster of Paris and (2) NH in amount from about 8 percent to about 23percent of the weight of water plus NH 2. The product of claim 1 towhich is added from about 0.2 to about 2% of the weight of said plasterof Paris, of a bonding agent selected from the class consisting ofcooked starch, dextrin, polyvinyl aceateemulsion, methyl cellulose,hydroxyethyl ethyl cellulose, hydroxypropyl methyl cellulose and mixturethereof.

3. The product of claim 2 to which there is further added from 0.5 to2.5% of the weight of said plaster of Paris, of K 4. A method of makinga plaster of Paris bandage which comprises the steps of (1) making aslurry of powdered plaster of Pari in water containing boric acid inamount from about 0.05 to about 1% of the weight of plaster of Paris andNH in amount from about 8% to about 23% of the weight of water plus NHwhile the plaster of Paris remains unset; (2) coating the product ofstep (1) upon a flexible inert carrier; and (3) drying the product ofstep (2) at a temperature of from about F. to about 250 F. to yield adry, settable plaster of Paris product.

5. The method of claim 4 wherein to the slurry of step (1) is added from0.2 to 2% of the weight of said plaster of Paris of bonding materialselected from the class consisting of cooked starch, dextrin, polyvinylacetate emulsion, methyl cellulose, hydroxyethyl ethyl cellulose,hydroxypropyl methyl cellulose and mixtures thereof.

6. The method of claim 5 wherein to the slurry of step (1) is furtheradded from 0.5 to 2.5% of the weight of said plaster of Paris of K 50 Noreferences cited.

HELEN M. MCCARTHY, Acting Primary Examiner. S. E. MO'IT, AssitantExaminer.

1. A SETTABLE PLASTER OF PARIS MIX COMPRISING MAJOR PROPORTIONS OFPOWDERED PLASTER OF PARIS AND WATER AND MINOR PROPORTIONS OF (1) BORICACID IN AMOUNT FROM ABOUT 0.05 TO ABOUT 1 PERCENT OF THE WEIGHT OFPLASTER OF PARIS AND (2) NH3 IN AMOUNT FROM ABOUT 8 PERCENT TO ABOUT 23PRECENT OF THE WEIGHT OF WATER PLUS NH3.